Subject: [Sigtran] M2PA questions on synchronization and message handling

From: Dario <[email-masked]>
Date: Mon, 17 Dec 2007 03:47:00 -0800 (PST)

HI,

here are three questions for M2PA RFC 4165:

1.	Synchronization after PO during Initial Alignment
the question is: is the synchronization in Initial Alignment identical to
synchronization in stable PO state in terms of exchanged messages and
procedures?
(at least – during Initial Alignment , there was no User Data messages sent,
so what is the purpose of such sequence?)

2.	when FLUSH primitive is received from MTP level 3 in M2PA, should M2PA:
a.	flush outgoing buffers
b.	flush both outgoing and incoming buffers?

Perhaps it is not clear are both points a) and b) listed in RFC section
4.1.4.  to be executed?

3.	Unexpected FSN and/or invalid BSN
The RFC states that 
„If M2PA receives a User Data message with an FSN that is out of order, M2PA
shall discard the message“. (RFC4165, section 4.2.1)

In addition to that, Q.703 states
„In the case that a message signal unit or fill-in signal unit is received
having a backward sequence number value which is not the same as the
previous one or one of the forward sequence number values of the signal
units available for transmission, the signal unit is discarded. The
following message signal unit or fill-in signal unit is discarded.
If any two backward sequence number values in three consequtively received
message signal units or fill-in signal units are not the same as the
previous one or any of the forward sequence number values of the signal
units in the retransmission buffer at the time that they are received, then
they are received, then level 3 is infomed that the link is faulty“
(section 5.3.1)

If M2PA is to follow all these statements that it would mean that reception
of message with invalid sequence number would cause transition to Out Of
Service state:

For example:
-	User Data message with either unexpected FSN or invalid BSN is received:
discard it
-	User Data with correct FSN (FSN=x) is received, it must be discarded due
to Q.703 statement
-	Nex User Data with FSN=x+1 is received. This is now unexpected FSN since
previously received message is discarded. Etc.

Best regards, Dario

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---

From: "Brian F. G. Bidulock" <[email-masked]>
Date: Mon, 17 Dec 2007 09:30:27 -0700

Dario,

Dario wrote:                                     (Mon, 17 Dec 2007 03:47:00)
> so what is the purpose of such sequence?)

Sequence numbers do not need synchronization during initial alignment
because they are still at their initial values.

> Perhaps it is not clear are both points a) and b) listed in RFC section

Yes, both.  Note that in (a) incoming buffers are not flushed, just messages
that were queued and unacknowledged, which are the messages received during
processor outage that could not be delivered to MTP3 from earlier on in
section 4.1.4.

> shall discard the message“. (RFC4165, section 4.2.1)

The statement in RFC 4165 section 4.2.1 applies to an FSN that is out of
order.

> previously received message is discarded. Etc.

The statement in Q.703 section 5.3.1 applies to a BSN that is invalid.

You compare apples and oranges.

--brian

-- 
Brian F. G. Bidulock
[email-masked]
http://www.openss7.org/

---

From: Dario <[email-masked]>
Date: Thu, 20 Dec 2007 04:29:35 -0800 (PST)

Hi,

Brian F. G. Bidulock wrote:
>> (at least â during Initial Alignment , there was no User Data messages
>> sent,

OF course the sequence numbers do not need synchronization since there were
no
User Data messages exchanged at that point.
But my questions are the following:
- is the sequence of messages for synchronization the same in Initial
Alignment as in stable PO state:
  LS Processor Recovered ->
  LS Ready (1) <-
  LS Ready (1) ->

- how these messages collide with Link Status messages (for example, LS
Ready on stream 0) and what are the consequences?

Brian F. G. Bidulock wrote:
>> âIf M2PA receives a User Data message with an FSN that is out of order,
>> M2PA
>> shall discard the messageâ. (RFC4165, section 4.2.1)
> 
>> âIn the case that a message signal unit or fill-in signal unit is
>> received
>> they are received, then level 3 is infomed that the link is faultyâ
>> (section 5.3.1)
>> 

not if they are connected:

1. message is received with invalid BSN. This message wil be discarded
2. User Data message is received with FSN=x, valid, but it should be
discarded according to Q.703 statement above.
3. User Data message is received with FSN=x+1. This is unexpected FSN and
the message will be discarded....and so on.

finally ack timer will expire and the link will go Out of Service.

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---

From: "Brian F. G. Bidulock" <[email-masked]>
Date: Thu, 20 Dec 2007 10:45:12 -0700

Dario,

Dario wrote:                                   (Thu, 20 Dec 2007 04:29:35)
> Ready on stream 0) and what are the consequences?

See test cases 3.1.11, 3.1.12, 3.1.13, 3.1.14, 3.1.15, 3.1.16, 3.1.35,
3.2.1, 3.2.2, 3.2.3, 3.2.4, 3.2.5, 3.2.6, 3.2.7 in the test suite
draft-bidulock-sigtran-m2pa-test-08.txt for examples.

> finally ack timer will expire and the link will go Out of Service.

The peer violated the protocol at 1.  (The rest is just one possible
result of that.)  You should not find it surprising that signalling
links fail to operate and eventually find themselves in the out-of-
service state when one side refuses to follow the protocol.

--brian

-- 
Brian F. G. Bidulock
[email-masked]
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---

From: "Brian F. G. Bidulock" <[email-masked]>
Date: Thu, 20 Dec 2007 10:57:57 -0700

Dario,

Brian F. G. Bidulock wrote:              (Thu, 20 Dec 2007 10:45:12)
> service state when one side refuses to follow the protocol.

See also test case 3.8.10-1 in draft-bidulock-sigtran-m2pa-test-08.txt
and section 2.2 of draft-bidulock-sigtran-ig-01.txt.

--brian

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Brian F. G. Bidulock
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